Push Buttons, Sensors & Switches (etc)

[cobra4202]

anyone ever use a proximity sensor to calculate rpm in a plc?

[TJ McDermott]

We've used discrete sensors (both prox and photoelectric) to do that.  What specifically are you looking for?

[cobra4202]

basically calculate Rpm based on detecting the teeth on a chain sprocket with a prox. sensor.

[TJ McDermott]

That's not a very good combination.  Even with some of the smaller inductive proximity sensors, you might have detection problems.  If the sensor barrel is parallel to the axis of rotation, the tooth curves away from the detection plain (so it can easily enter the chain).  If the sensor barrel is perpendicular to the axis of rotation, you're detecting on the narrowest part of the tooth (tip instead of root).  Even if you detect it reliably, at higher rotational speeds you might pass by the sensor before the PLC can register it.

For instance, assume a #40 sprocket, 24 teeth (15 degrees per tooth/pocket cycle).  The width of the tooth just above the root covers 8.38 degrees.  This makes a decent target for a fiber optic through-beam photoelectic sensor (1mm diameter fiber tips).  The tip of the tooth spans just 1.13 degrees.  If you detect the tooth from the perpendicular orientation, the sensor has just 1.13 degrees out of 15 to detect the tooth.  As speeds increase, you start to dip below repsonse time of sensor and PLC very quickly.

Grab some real numbers:  60RPM sprocket speed, #40 sprocket, 24 teeth.  60rpm is 1 revolution per second, or 24 teeth per second, or .0417 seconds per tooth.  If just 1.13 degrees out of 15 degrees triggers the sensor, the actual target material (tip of the tooth) is in front of the sensor for just .0031 seconds (3.1 milliseconds).  Most Turck sensors report a 0.3 to 1.0 millisecond response time, but you're definitely going to have to worry about PLC scan times, and this is just at 60rpm.

You've not shared any hard information (real sprocket parameters, rotational speed, how accurate, etc.)  Depending on your application, you may not need to detect each tooth.  For higher speeds you can detect a set screw instead (well, cap screw head so you'll have a bigger target), and count actual roations.  Maybe you put 4 screws in the face of the sprocket (again, cap screws for larger target).  Maybe you go the fiber optic sensor route to detect the teeth near the root.

When we do this sort of application, we frequently make our own "shutter" wheels bolted to the sprocket to deal with these problems.

Does this help, or were you asking about the ladder rungs dealing with calculating RPM based on prox. sensor input?

[fordtt]

Since you're only using it for RPM measurement, we typically use a small screw head or keyway on the shaft.  In some applications, we've welded a small nub on the shaft for pick up.  The detection resolution on a prox with gearheads or sprockets isn't sharp enough and the faster the gear turns, the more skewed your signal and you'll actually miss counts.  We have even drilled and tapped a small hole IN a sprocket sticking out to the side for a shielded prox to pick up.  With a sprocket you'll have to calculate the RPM, with only one pickup, it's very easy to calculate the RPM.

[cobra4202]

Yes, Thanks that does help , but I was more interested in the logic of calculating the rpm.

[Smoke]

This flag was done with timer set to 1 second and the flag time was 100ms, the RPM should be 60 rpm minus the flag on time.

Message Edited by Smoke on 09-22-2009 06:25 PM

[cobra4202]

The Flag is the sensor input? Where did get the values for the timer?  Thanks!

[Ken_Roach]

Many logic controllers include an onboard hardware counter or interrupt function that makes functions like RPM monitoring easier to perform.

What controller family and hardware are you using ?

[Smoke]

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cobra4202 wrote:
The Flag is the sensor input? Where did get the values for the timer?  Thanks!

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Yes the flag is the sensor input. 1 flag per rotation. The timmer value just needs to be longer than the time it takes to make 1 rotation.

Ken, I would to learn about hardware counter in the CLX family, or are you talking about rack additions like high speed counters etc?

Message Edited by Smoke on 09-23-2009 02:41 PM